1. Field of the Invention
The present invention relates to methods and apparatus reading the information written on a magneto-optical writing media.
2. Discussion of the Related Art
A magneto-optical writing media has been in practical use as information writing media which is possible to rewrite on the writing media with a high density. In particular, the writing media having a writing layer made of the amorphous alloy of rare-earth and transition metals have a remarkable characteristic.
The explanation for an example of writing information on the magneto-optical writing media is as follows. By focusing a laser beam on the surface of a magneto-optical writing media as a spot whose diameter is as short as the wavelength of the light, the temperature of the writing layer is raised up to 150.degree. C.-200.degree. C. Thus, if the temperature of the writing media heated by the laser beam goes up to Curie temperature(Tc), the magnetization is disappearing. At this time, if constant magnetic bias field is applied in one direction by a magnet, a magnetic mark(or a pit) is written on the writing layer by the magnetic inversion occurring when the heated area returns to the room temperature.
Referring to FIGS. 1 and 2, the process for writing the information on the magnetooptical writing media is explained. FIG. 1 is a diagram showing a conventional writing apparatus writing the information on the magneto-optical writing media. FIG. 2 is a timing diagram to explain the operation of the apparatus in FIG. 1. Based on the information initially preformatted on the optical disk, a channel clock signal generator 9 generates a channel clock signal 10. According to the channel clock signal 10, laser driver 11 makes the laser diode 1 emit a pulse beam. This laser light 2 is irradiated on the optical disk 8 as an optical spot 4 by an objective lens 3. On the other hand, through a magnetic head 5 closely disposed to optical disk 8, a data signal generator 6 generates a modulation magnetic field 7. As shown in FIGS. 2(a) to 2(e), if the frequency of channel clock signal 10 is increased and the laser beam focussed as an optical spot 4 is irradiated on the optical disk 8 as a pulse type, according to the combination of the pulse type laser beam and the modulation magnetic field, the optical spot 4 in synchronization with the channel clock signal 10 is irradiated on the optical disk 8. In response to the optical spot irradiated like this, marks are overlapped and then written on the optical disk 8. According to this method, the writing of the magnetic mark which has a mark length shorter than the optical spot 4 is done. This method is a known technique published in Japanese patent publication Pyungsung No 1-292603.
On the other hand, as a method for reading the written information on the optical disk, a method for focusing a laser beam with a constant output power as a spot whose diameter is as short as its wavelength and then for irradiating the beam spot on the surface of the magnetooptical writing media is well known.
The focussed optical spot is reflected from the surface of the magneto-optical writing media. At this time, the polarization state of the laser beam is changed by Kerr effect. By optically detecting the change of the polarization state of the reflection light, the information written on the magneto-optical writing media in magnetic state is read from the writing media. However, as shown in FIG. 3, as the information is written on the magneto-optical writing media in high density, the length of the magnetic mark is getting shorter and the optical spot is longer than the magnetic pit(or mark). As the result, a problem arises in the resolution capability when the mark is read.
In order to solve this problem, super resolution techniques have been attempted. As one of the techniques, a method of magnetically induced super resolution(MSR) using an exchange coupling force has been introduced. A method of using an in-plane magnetic layer which is a kind of the MSR is shown in FIG. 4. As shown in FIG. 4, the magneto-optical writing media consists of two layers having an exchange coupling structure between a reading layer with a relatively low coercivity and a writing layer with a relatively high coercivity. The reading layer has the in-plane magnetic layer. However, when the temperature of the layer is over a specific temperature, the magnetic orientation of the reading layer changes and becomes perpendicular magnetization. The writing layer is a perpendicular magnetization layer so as to maintain the information. If the laser beam is irradiated on the reading layer in order to read the information, the magnetization of the reading layer of high temperature area in the middle of the optical spot (the area with the temperature above threshold value in FIG. 4) is changed from the in-plane magnetization to the perpendicular magnetization, and then a polar Kerr effect comes out. In other words, the magnetic field in the high temperature area of the reading layer is changed into the direction of the magnetic field of the writing layer. On the contrary, because the Kerr effect does not occur in the low temperature area in the neighborhood, the magnetization of the writing layer is masked. Therefore, if the power of the reading laser beam is properly selected, the written information is read from the high temperature area corresponding to the middle of the laser spot, and, as the result, the reading operation in super resolution is possible.
However, because the method of reading a small magnetic pit(or mark) by masking the reading layer like this uses a subtle temperature distribution in the light spot, the change in the magnetic orientation is affected by the fluctuation of the rotation speed of the laser disk and the change in the power of the reading laser beam, and therefore is unsatisfactory As the result, a good carrier-to-noise ratio is not obtained. Therefore, the error rate becomes high and the jitter occurs, and a good quality of the readout signal is not obtained.
As a method to solve this problem, the technique irradiating a reading laser beam in a pulse synchronized with the channel clock signal is disclosed in Japanese patent publication Pyungsung 4-325948. According to this technique, there is a merit making the error rate very low.
FIG. 5 shows an example of the apparatus reading the information written on the optical writing media. FIG. 6 is a timing diagram to explain the operation of the apparatus in FIG. 5. In this reading apparatus, a method irradiating a laser light of a pulse as a reading light is adopted. Based on the reading clock signal of clock generator 58 shown in FIG. 6, a pulse shaper 57 outputs a signal of the pulse type. Laser driver 56 in response to this pulse signal drives the laser diode 55. As shown in FIG. 6, the laser beam emitted from the laser diode 55 in the pulse type is focussed on the surface of the optical writing media 51 by a collimator lens 54 and an objective lens 52. The laser beam spot focussed on the optical writing media 51 is reflected from the reading layer and passed through the objective lens 52, and then comes toward the first polarized beam splitter 53. The optical spot is again applied to the second polarized beam splitter 59 by the first polarized beam splitter 53. In this splitter 59, p-polarization component of the light spot is transmitted through the splitter 59 and s-polarization component is reflected from the splitter 59.
The p-polarization component and the s-polarization component are focussed and then converted into electrical signals by the first photo detector 61 and the second photo detector 60, respectively. The photoelectric converted electrical signals are applied to the difference amplifier 62, are amplified and are outputted as a signal shown in FIG. 6(d). The readout signal processor 63 processes the output signal of the difference amplifier 62 and then generates a bit signal corresponding to the detected information written on the media, that is, a binary signal. FIG. 6 shows the written marks written on the optical writing media 51. In FIG. 6(c), the hatched marks are the binary signals of high level and the white marks are the binary signals of low level.
As described above in detail, when irradiating the pulse type of laser beam on the optical disk and reading the information written on the magneto-optical writing media, the laser beam spot of the pulse type may not be exactly focussed on the written mark because of the fluctuation of the magneto-optical disk and the external disturbances. In this case, as in A region of FIG. 6, the laser beam spot of the reading light is irradiated over the two magnetic domains, i.e., two written marks having different levels to each other. Therefore, the regeneration signal from the magneto-optical writing media is not detected. B region in FIG. 6 shows the case for the laser beam spot being normally irradiated on the only one written mark.